Food and drink products and other perishable items are often packaged in tubular containers, which are sealed at both ends. These tubular containers typically include at least one structural body ply and are formed by wrapping a continuous strip of body ply material around a mandrel of a desired shape to create a tubular structure. The body ply strip may be spirally wound around the mandrel or passed through a series of forming elements so as to be wrapped in a convolute shape around the mandrel. At the downstream end of the mandrel, the tube is cut into discrete lengths and is then fitted with end closures to form the container.
Tubular containers of this type typically include a liner ply on the inner surface of the paperboard body ply. The liner ply prevents liquids, such as juice, from leaking out of the container and also prevents liquids from entering the container and possibly contaminating the food product contained therein. Preferably, the liner ply is also resistant to the passage of gasses, such as oxygen and nitrogen, so as to prevent odors of the food product in the container from escaping and to prevent atmospheric air from entering the container and spoiling the food product. Thus, the liner ply provides barrier properties and the body ply provides structural properties.
In addition, current commercial containers often have membrane-type lids or end closures heat sealed to a curled or bead-shaped rim of the composite container wall to form a peelable seal. The rim is formed by turning outwardly the end of the container to position the inner layer of the liner material on the outwardly curved surface.
A major difficulty in developing a usable heat seal between the container lid and the rim of the container wall is balancing bond strength with ease of opening for the end user. During transport, the sealed containers experience temperature and pressure extremes that stress the heat seal and can lead to rupturing of the container. The bond strength must be sufficient to withstand the rigors of transportation. In particular, when containers packaged and sealed at one elevation are then subjected to lower ambient air pressure, such as during air transportation or when transported to consumers at higher elevations, a relative positive pressure is created within the container which could cause the seal between the lid and the container to rupture. This ability of the container to avoid rupturing under such conditions is known as burst strength. However, as the burst strength increases, there is generally a concomitant increase in difficulty of opening of the container, which is exhibited by the peel strength or peel resistance of the container. The higher burst strength indiscriminately prevents both rupturing during transport and opening by the end user.
Certain types of heat sealable coatings have been used in both the lidding and liners of conventional containers. For example, SURLYN.RTM. polymer, a product of Dupont, is a material known in the art and is commonly used as a heat seal coating. SURLYN.RTM. polymer is an ionically cross-linked polymer with limited flow characteristics when heated. Typically, the layer of the container and the layer of the membrane which contact each other are constructed of SURLYN.RTM. polymer, and may be coated with a wax. These two layers of SURLYN.RTM. polymer are heat sealed along the top surface of the container bead. The two SURLYN.RTM. polymer layers create an extremely strong bond layer that remains relatively uniform in thickness across the seal area. Due to the strong cross-linked bond created by SURLYN.RTM. polymer, however, opening the container can require a peel force which is too high for some consumers and usually results in tearing and exposure of the other layers of the container wall, such as the paperboard body wall, as is illustrated in U.S. Pat. No. 4,280,653 to Elias. This gives the top of the container a ragged, undesirable appearance.
In the parent application, Ser. No. 09/065,783, the formation of two heat seal beads is described. The two beads comprise an inner heat seal bead and an outer heat seal bead, each heat seal bead being formed of the heat sealable polymers of the seal layers of the membrane and the liner. The two beads are formed by using heat and pressure to force the heat sealable polymers to flow away from the central portion of the heat seal area and towards the interior and exterior of the container. The reduction in the amount of heat seal material in the central heat seal area reduces the bond strength in the central heat seal area and allows opening of the container without unsightly tearing of the liner and exposure of the paperboard layer of the container wall. However, formation of the beads according to the parent application does not entirely erase the difficulty of balancing burst strength and ease of opening.
It would be advantageous to provide a sealed container and method for sealing a container that combine improved ease of opening and an attractive appearance after opening with the seal strength and barrier properties required for protection of the products within the container.